360 



Plant Physiology 



pollinated and cross-fertilized when fruit is produced, 

 because in these species stamens and pistils are on distinct 

 individuals. In corn there is opportunity for self-fer- 



FIG. 102. An ear from an isolated stalk of corn ; infertility from lack of 



cross-pollination. 



tilization, but crossing is the rule. In fact, isolated stalks 

 of corn seldom set more than scattering grains (Fig. 102). 

 Darwin's observation respecting the necessity of cross- 

 pollination in red clover has become a familiar instance 

 among perfect flowers. He demonstrated that the heads 

 of this species protected from bees and other insects set 

 no seed. This may not be due, in the case of clover, to 

 the ineffectiveness of the pollen of each particular blossom 



